feature request: `#[backtrace]` attr add force option#321

currently the `#[backtrace]` attr has no option, and `thiserror` generated codes always use `Backtrace::capture()` however, if user want some error always enable backtrace, they have to - write `impl From<Err> Error` manually - or set env `RUST_BACKTRACE=1`, but that will cause all backtraces are enabled, even they only want to enable a few error backtrace if `#[backtrace]` add a new option `force`, like `#[backtrace(force=true)]`, when user enable the force capture, `thiserror` generated codes can use `Backtrace::force_capture()` instead of `Backtrace::capture()`